Theoretical Study On The Mechanical And Thermodynamic Properties Of Hf_0.25Ti_0.25Zr_0.25Sc_0.25-xAl_x

With the deepening of the research,the development of metal materials gradually faces a bottleneck.In traditional alloys,the space for development and research is becoming smaller and smaller,and material researchers are constantly looking for a breakthrough.Therefore,the high-entropy alloy is proposed as a new alloy design concept,and its unique structural characteristics and excellent performance have attracted the attention of a large number of researchers.Up to now,most of high-entropy alloys have FCC and BCC structures,while the hexagonal close packed?HCP?high-entropy alloy is very rare and mainly composed of rare earth elements,so the development of more HCP high-entropy alloy materials with less and without rare earth elements has become a hot spot.Although experimental research on the high entropy alloys has made great achievements,the traditional experimental study is facing with great difficulty due to complex multicomponent and disorder of the solid solution structure of high entropy alloys.And theoretical simulation and calculation can be convenient and effective to predict material structure,properties and application environment,would provide theoretical guidance for the optimal design and application of new high entropy material in the future.Therefore,on the basis of density functional theory?DFT?in combination with the coherent potential approximation?CPA?,this paper studied the mechanical properties of HCP highentropy alloy Hf_0.25Ti_0.25Zr_0.25Sc_0.25-xAl_x?x ? 15 at.%?in the elastic range by ab initio calculation.And the thermodynamic properties of Hf_0.25Ti_0.25Zr_0.25Sc_0.25-xAl_x?x ? 15 at.%?were studied by EMTO-CPA ab initio calculation combined with quasi-harmonic Debye-Grüneisen model.The main research contents are as follows:The structure and mechanical properties of a new HCP high entropy alloy Hf_0.25Ti_0.25Zr_0.25Sc_0.25-xAl_x?x ? 15 at.%?were studied by ab initio calculation based on the coherent potential approximation?CPA?.The calculated enthalpy of formation shows that the HCP high entropy alloys Hf_0.25Ti_0.25Zr_0.25Sc_0.25-xAl_x?x ? 15 at.%?are thermodynamic stable.The calculated elastic constant satisfies the mechanical stability criterion of hexangular crystal,so HCP Hf_0.25Ti_0.25Zr_0.25Sc_0.25-xAl_x?x ? 15 at.%?are mechanical stability.The electronic structure reveals the covalent bond formation and underlying mechanism for high stability of Hf_0.25Ti_0.25Zr_0.25Sc_0.25-xAl_x?x ? 15 at.%?.In the elastic range,the strength and hardness of HCP high-entropy alloys Hf_0.25Ti_0.25Zr_0.25Sc_0.25-xAl_x?x ? 15 at.%?basically meet the mixing rules,which means that the strengthening effect of HCP high-entropy alloy is not obvious in the small elastic range.It is noted that the increase of Al content is beneficial to the strengthening effect of Hf_0.25Ti_0.25Zr_0.25Sc_0.25-xAl_x?x ? 15 at.%?high-entropy alloy.In addition,the analysis of brittleness and ductility showed that Hf_0.25Ti_0.25Zr_0.25Sc_0.25-xAl_x?x ?15at.%?had good ductility.The increase of Al content can improve the malleability of Hf_0.25Ti_0.25Zr_0.25Sc_0.25-xAl_x?x ? 15 at.%?.The anisotropy analysis showed that HCP high entropy alloy Hf_0.25Ti_0.25Zr_0.25Sc_0.25-xAl_x?x ? 15 at.%?showed little anisotropy,and the Al content had little influence on the anisotropy.These results are valuable for understanding the mechanical behavior of high entropy alloys and their application.The thermodynamic properties of Hf_0.25Ti_0.25Zr_0.25Sc_0.25-xAl_x?x ? 15 at.%?are studied by ab initio method in conjunction with the quasi harmonic DebyeGrüneisen model.It is worth noting that the substitution of Al for Sc has a significant effect on the structure and thermodynamic properties of HCP Hf_0.25Ti_0.25Zr_0.25Sc_0.25-xAl_x?x ? 15 at.%?.With the increase of aluminum content,although the softening trend is similar,the volume modulus increases obviously,so the addition of aluminum is beneficial to the engineering application.The thermal expansion coefficient of Hf_0.25Ti_0.25Zr_0.25Sc_0.25-xAl_x?x ? 15 at.%?high entropy alloy increases obviously with the increase of aluminum content,so the aluminum content can control the thermal expansion behavior of the alloy.Al content has little effect on CV and CP of HCP Hf0.25Ti0.25Zr0.25Sc0.25-x Alx.Due to the high temperature softening,the Debye temperature ?D decreased with the increase of temperature,although the increase of Al content made the Debye temperature ?D increase significantly.The Grüneisen parameter ? of Hf0.25Ti0.25Zr0.25Sc0.25-x Alx increases with increase of temperature and aluminum content.Although the entropy change behavior is similar,the increase of Al content slightly inhibits the entropy increase trend of HCP high-entropy alloy Hf0.25Ti0.25Zr0.25Sc0.25-x Alx?x ? 15 at.%?,so the high aluminum content is not conducive to the formation of solid-solution high-entropy alloy.The research results are of great significance to the overall performance optimization and design of Hf0.25Ti0.25Zr0.25Sc0.25-x Alx alloy.